Cellular radio systems comprise a plurality of base stations connected to a switched telecommunications network and a plurality of mobile units. Mobile units are connected to the telecommunications network by establishing radio links with nearby base stations. As mobile units travel from an area served by a first base station to an area served by a second base station, the radio link between the mobile unit and the first base station must be replaced by a radio link between the mobile unit and the second base station to maintain communications between the mobile unit and the telecommunications network. This operation is generally called a "hand-off" of the mobile unit from the first base station to the second base station.
Hand-off operations are triggered when the base station currently linked to a mobile unit detects degradation of radio signals received from that mobile unit. Consequently, base stations must monitor the quality of signals received from mobile units. One useful measure of signal quality is the carrier-to-interference ratio of the received signals.
Each base station receives radio signals from all mobile units which are operating within its receiving range. As this receiving range overlaps with serving areas of neighboring base stations, the base stations must be able to distinguish signals received from those mobile units they are currently serving from signals received from mobile units served by neighboring base stations. To this end, each base station transmits an out-of-band Supervisory Audio Tone (SAT) to each of the mobile units it is currently serving, and each mobile unit retransmits the SAT for reception by the base stations. As adjacent base stations are assigned distinct SATs, each base station can recognize signals intended for its reception by the presence of its distinctive SAT in those signals.